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M/XDR Mycobacterium tuberculosis
Mycobacterium Tuberculosis Overview This species of bacteria a nonmotile rod approximately 2-4 micrometers in length and 0.2-0.5 micrometers in width. It contains my colic acid in its cell wall and therefore requires a special "acid-fast" process vs. traditional gram staining procedures in order to take up a stain (aka not considered gram positive or gram negative). As an obligate aerobe, this species requires high levels of oxygen in order to survive and replicate (1). Therefore when it infects it's host (humans) it travels to the the upper lobes of the lungs which have access to high oxygen levels. In humans, M. tuberculosis is considered the primary etiological agent of tuberculosis. In the intitial exposure to TB (primary) typical symptoms present are a minor cough and fever that leads to chest pain, difficulty breathing, wheezing and coughing up blood. This is due to the bacteria invading and surviving in unactivated alv eolar macrophages within the lungs. This cells can become surrounded by other immune cells in the body leading to tubercules, or darkened regions within X rays of the lungs. This tubercules can become calicfied as well when the cells die (Gohn complexes) which leads to further difficulty breathing. Death can result without proper antibiotic treatment due to massive lung damage that leads to hypoxia in patients. The infection can become latent within the alveolar cells and lead to secondary /recurrent TB. The cause of the symptoms is normally due to host cell damage by the bacteria as well as mass immune cell responses (2). Drug Resistant Tuberculosis First line antibiotics such as combination therapy of isoniazid and rifapentine (rifampin derived) are used daily for 3-6 months to treat and cure TB in most cases. However M. tuberculosis has shown varying degrees of antibiotic resistance in humans. MultiDrug Resistant (MDR) TB: MDR-TB is resistant to first line drugs mentioned above (isoniazid and rifampin) and can lead to a mortality rate of around 30% (2). Extensively Drug Resistant (XDR) TB: XDR-TB is resistant to rifampicin, isoniazid, quinolones as well as some second-line drugs including kanamycin, amikacin, and capreomycin. The mortality rate for this form of TB is around 60% (2). Totally Drug Resistant (TDR) TB: TDR-TB is resistant to all first and second line drugs and has a characteristic round cell shape with very thick cell walls when viewed under the microscope. The mortality rate for this form of TB could potentially reach 100% (2). Genetic Analysis of M/XDR TB In order to combat the drug resistant strains of M. tuberculosis ''it is necessary to know that a resistant strain is in fact prevalent in the patient upon diagnosis with the infection to assign the proper antibiotic treatment course from the start. This would give the patients a higher chance of survival and minimize cases of resistance since they are being treated with the correct line of antibiotics that the bacteria would be considered susceptible to. In a study conducted through the Global Consortium for Drug-Resisant TB Diagnostics, 417 MDR/XDR-TB cases were analysed to determine if it was possible to predict the phenotypes of the strains of resistant bacteria through looking at genetic mutations among the strains (3). The TB samples were taken from the patients in areas of the world with high levels are drug resistance such as Moldova, the Philippines, South Africa, and India. These isolates were then tested using standardized phenotypic drug susceptibility testing (growth indicates resistance, no growth indicates sensitivity) for a multitude of different antibiotics used to treat TB. The results from these tests indicated that 370 of the isolates were isoniazid (INH) resistant, 356 were rifampin (RIF) resistant, 292 were moxifoxacin/ofoxacin (MOX/OFX) resistant, 230 were amikacin (AMK) ressitant, 219 were capreomycin (CAP) resistant, and 286 were kanamycin (KAN) resistant (3). Next 8 different genes (''katG, inhA, rpoB, gyrA, gyrB, rrs, eis, ''and ''tlyA) assumed to be related to varying drug susceptibility in the strains were sequenced using Sanger sequncing methods. Results indicated that resistance to INH were due to 4 SNP differences within the ''katG/inhA ''genes with 96% sensitivity and 97-100% specificity. Resistance to RIF was concluded to be caused by 11 different SNP variations within the ''rpoB ''with a combined sensitivity of 98%. 8 SNPs within ''gyrA ''were responsible to resistance to MOX/OFX with 90% sensitivity. The rrs gene contained 2 SNPs that concluded resistance against AMK/CAP with 89-90% sensitivity and 71% sensitivity for KAN resistance. However when the ''eis ''gene was taken into account resistance sensitivity was increased to 93% for AMK and 91% for KAN. This information concludes that specific SNPs from the resulting sequences in each gene concluded that phenotypic resistance in MDR/XDR-TB to INH, RIF, FQ, AMK, KAN, and CAP could be predicted based on 30 SNP differences within 6 genes with 90-98% sensitivity and almost 100% specificity in most regions (3). References 1. Todar, Kenneth. "Mycobacterium Tuberculosis and Tuberculosis." Todar's Online Textbook of Bacteriology. Kenneth Todar, 2012. 2. Dr. Doug Johnson, Clinical Microbiology Lecture Notes- Mycobacterium, Spring 2014, UVM MMG Dept. 3. Rodwell, Timothy C. "Predicting Extensively Drug-Resistant Mycobacterium Tuberculosis Phenotypes with Genetic Mutations." US National Library of Medicine. Journal of Clinical Microbiology, Mar. 2014. Pubmed ID: PMC3957771